1. Field of the Invention
The invention relates to communication signal processing and, more particularly, to a method and an arrangement for determining the propagation delay of a signal between a first and second station in the arrangement with to determine the spatial distance between the stations.
2. Description of the Related Art
The industrial domain is an instance of where precisely determining a radio transmitter's position or, as the case may be, its distance from a base station or such is of significance. Besides meeting the demand for cost- and energy-saving measuring systems, particularly for applications in closed spaces or halls it is therein necessary owing to possible multipath reflections to employ high-resolution measuring systems to obviate errors in distance measuring. For instance, Ultra Wide Band (UWB) signals offer a large signal bandwidth and thereby promise a relatively high resolution and high degree of accuracy.
Various methods that employ, for example, optical signals, ultrasound signals, or radio sensor technology, are known for determining a position or, as the case may be, distance. In particular, the methods for determining a distance bistatically with the aid of radio signals can be divided into three categories.
The first category is communication-based systems. Here, the distance is determined additionally from the communication signal. The degrees of accuracy achievable in distance measurements are not expected to be high because less stringent demands are placed on synchronizing in many communication systems or, as the case may be, only a very narrowband radio channel is available.
The second category is Frequency Modulated Continuous Frequency-Stepped Continuous Wave (FMCW/FSCW) solutions. These systems operate in the Industrial, Scientific, and Medical (ISM) bands and enable a distance value to be determined in a manner similar to classical FMCW radar by tuning a transmitting frequency. Transponder-based or, as the case may be, backscatter solutions are therein employed on the one hand and, on the other, receivers that can synchronize themselves therewith.
The third category is ultra wideband (UWB) systems. These systems exploit new regulations allowing the emission of ultra wideband signals which, though, have a very low spectral power density. The receiver architectures can be, for example, power detectors having a moderate capacity or coherent receivers that require either very long correlation times or an extremely fast sampling rate.
These solutions have cost implications specifically for one-dimensional distance measuring between two radio stations because complex signal-processing steps are necessary in at least one of the two stations.
FIG. 1A shows a conventional principle of measuring distance between a transmitting/receiving station 10 and an object 20. Transmitting/receiving station 10 has a phase-locked loop (PLL) 11 that drives a first 12 and second pulse generator 13. First pulse generator 12 thereupon generates a first series TX-series of signal pulses TX-puls(i), where i=0, 1, 2, 3, . . . , at a pulse-repetition rate f0. Second pulse generator 13 correspondingly generates a second series RX-series of signal pulses RX-puls(j), where j=0, 1, 2, 3, . . . , at a pulse-repetition rate f0+Δf. It can therein be assumed that, for example, signal pulses TX-puls(0) and RX-puls(0) are generated simultaneously.
Because pulse-repetition rates differ by the amount Δf, signal pulses of first series TX-series and second series RX-series regularly coincide at time intervals 1/Δf, meaning at instants TTX/RX(a)=a*1/Δf+c, where a=0, 1, 2, 3 . . . c is therein only a constant that depends on how instant t=0 is established (for example c=0 in the case of FIG. 1B).
First series TX-series is then emitted by an antenna 14 of transmitting/receiving station 10 and reflected on object 20 whose distance from transmitting/receiving station 10 is to be ascertained.
First series TX-series or, as the case may be, its signal pulses TX-puls(i) is/are reflected on object 20 and received by a second antenna 15 of transmitting/receiving station 10. Another series REC-series of signal pulses REC-puls(i) which, like first series TX-series, has a pulse-repetition rate f0, but which is time-shifted relative to series TX-series in keeping with signal propagation delay τ, will thus be available at antenna 15. Signal propagation delay τ is therein defined such that τ indicates the length of time a signal takes to travel from transmitting/receiving station 10 to object 20 and back. Received series REC-series has accordingly been shifted by τ relative to transmitted series TX-series.
Receive signal RX-series having pulse-repetition rate f0 is fed to a mixer 16 in which it is correlated with second series RX-series of signal pulses RX-puls(j) at a pulse-repetition rate f0+Δf. The signal will be detectable in the baseband if the two series' signal pulses coincide, meaning that a series BB-series having signal pulses BB-puls(k) and then having in each case a signal pulse BB-puls(k) will be generated in mixer 16 if a signal pulse RX-puls(j) of RX-series coincides in time with a signal pulse REC-puls(i) of REC-series. That occurs at instants TREC/RX(b)=b*1/Δf−τ*f0/Δf+c, where b=0, 1, 2, 3, . . . and c=constant (see above).
The time distance Δ between an instant TTX/RX(a) at which two pulses of series TX-series and RX-series coincide in time and an instant TREC/TX(b) at which two pulses of series of REC-series and RX-series coincide in time, Δ=τ*f0/Δf, will then with a=b be proportional to propagation delay τ and hence also to the distance between transmitting/receiving station 10 and object 20.
FIG. 1B shows the time characteristics of the different pulse series TX-series, RX-series, RX-series, and BB-series. Signal pulses TX-puls(0) and RX-puls(0) were generated simultaneously. Receive signal REC-series has been time-shifted relative to first pulse series TX-series due to propagation delay τ. The result BB-series for series REC-series and RX-series correlating that occur in mixer 16 is shown at the bottom of the chart. Comparing instant TREC/RX(1) with TTX/RX(1) will supply signal propagation delay τ. It should be appreciated that the same applies to comparing instants TREC/RX(2) with TTX/RX(2), etc., meaning that comparing instants TREC/RX(a) with TTX/RX(a) will supply the propagation delay being sought.
The conventional method that is presented assumes, however, that the transmitter and receiver are accommodated in one and the same station or, as the case may be, that a common time scale for the different signal series is established by PLL 11, meaning that the transmitter and receiver are synchronized. Only the distance from an object can accordingly be measured with this method, not the distance between two stations. If the transmitter and receiver are mutually separate and not fed by a shared PLL, for example, synchronizing will first need to be established for measuring the distance between the transmitter and receiver.